1. Field of the Invention
The present invention relates to the field of cargo handling, and in particular, to the transportation, loading and unloading of stacks of bagged cargo, such as, for example, stacks of sacks of grain.
2. Background of the Invention
Many types of cargo, such as grains, animal feed, seeds, meal and flour, and other particulate materials, are shipped in sacks. In the past, such sacks have been made of materials such as burlap and polyester. Although the sacks vary in size, they have commonly been of about 33 inches in length by 18-19 inches in width by 7-8xc2xd inches in thickness for 50 kg sacks. 25 kg sacks are of similar length and width, with reduced thickness. The product for which these bags are used may include wheat, corn, wheat flour, lentils, beans, animal feed, feed ingredients, as well as other particulate materials such as plastic pellets and fertilizers.
Sacks made of kraft paper, typically lined with a plastic material, have come into more prominent use for shipping products. Similar products to those described above may be shipped in the kraft bags but they are additionally suited to fine particular materials such as milk replacement products, cement, chemicals, food ingredients, powdered milk, corn and soy products, animal feed, seeds and the like. The 25 kg kraft paper sacks typically have dimensions of 28 inches in length by 16 inches in width with a thickness of 4-6 inches.
While there have been significant advances in the methods employed for the loading and unloading of vessels, the loading of bagged cargo is labor-intensive and time consuming, and labor costs for loading and unloading the vessels can be substantial. In addition, inefficient loading of a ship can increase transportation costs, as demurrage charges and the costs of owning and operating a vessel continue to be incurred while the ship is docked.
Another measure of efficiency in loading of ships is the stowage factor. The stowage factor may be calculated as of the volume occupied by the cargo divided by the mass of the cargo, such as cubic feet per ton. If sacks of cargo are loaded haphazardly into the hold of a vessel, it is inevitable that space will be wasted, and fewer tons of cargo will be stowed in a given volume.
In order to reduce the time a ship spends at the dock, sacks of products are frequently preslung, and stacks of such preslung sacks of products are lifted aboard the ship by the ship""s gear or by a crane. One efficient method for doing this is disclosed in my U.S. Pat. No. 4,737,069, which is incorporated herein by this reference. The stacks of sacks may be preslung at any location, including aboard barges or trucks, at warehouses, and at the dock.
The stacks of sacks are typically formed of successive layers of four sacks lying side by side with their edges slightly overlapping. The sling extends under the bottom layer of sacks about midway between their ends. The ship""s gear or crane may lift multiples of such stacks at a time using a spreader bar or the like, and may deposit them in the hold of the vessel in their stowage locations. The slings may be withdrawn from under the sacks, or may be left in place to facilitate unloading of the sacks from the ship. If a top strap is used across the top of the stack of cartons, of course, it must be released before the slings can be withdrawn.
Typically, cargo slings are made of a length of webbing with loops at each end. The loops are placed over hooks on the ship""s gear for lifting cargo into and out of the hold of a ship. As mentioned above, multiple stacks of sacks of products may be lifted into the hold of a ship by using a spreader bar. This is a beam or truss that may be attached to the ship""s gear or crane that carries multiple hooks along its length.
One problem that affects the stowage factor is the girdling of the sacks in the bottom layer. Since the products shipped in the sacks are generally particulate, and since the sacks are squeezed as they are lifted by a single strap of webbing, there is a tendency for the material to flow to the ends of the sack, and for the ends to bulge and sag downward. The bulging and sagging may occur to a lesser extent in layers of sacks above the lower layer. When the stack of sacks is landed in the ship, the result may be that the sacks forming the stack do not fully relax, resulting in the stack having a greater overall height, with an attendant increase in the stowage factor.
In addition, due to the material from which they are made and the relatively lower thickness of the sacks of product, the girdling problem is particularly acute for stacks of kraft paper sacks. In addition to the increase in stowage factor, moreover, the more-pronounced girdling of the kraft paper sacks and forces encountered when the stack of sacks are landed may result in tearing or rupture of the sacks, which is particularly undesirable when they contain food products. The problems are sufficiently pronounced with the kraft paper sacks that stacks of such sacks of products are at present viewed as not generally amenable to preslinging and loading using a single sling per stack.
The use of multiple slings per stack of sacks is considered undesirable, owing to the additional time required to presling the stacks. In addition, the time required to position an additional pair of loops per stack over the hooks of a spreader bar, and to remove them from the hooks is considered undesirable as it adds delay in the cycling of the ship""s gear or crane that decreasing productivity.
In one aspect, the present invention provides a cargo sling having loops at its two ends and providing at least two strap portions for supporting stacks of sacks of products or other cargo. In another aspect of the invention a cross strap is provided for securing across the top of a stack of sacks of products or other cargo. In yet another aspect of the invention, adjacent stacks of sacks of cartons are unitized by one or more bands of strapping extending lengthwise beneath and above sacks forming the adjacent stacks of sacks of products. In another aspect of the invention, a method of loading a ship is provided in which cargo slings having at least two strap portions for supporting stacks of sacks of products or other cargo are used in the loading and unloading of a vessel.